1) Field of the Invention
This invention relates to a valve operating system for driving an intake valve or an exhaust valve provided for an engine to open or close in response to rotation of a crankshaft, and more particularly to a valve operating system structure with a variable valve timing mechanism which can change over the operation timing of such valve between a low speed operation and a high speed operation.
2) Description of the Related Art
In recent years, as the automobile society has matured, the requirement for an engine has become progressively higher and is diversified, and much effort has been directed, in addition to the performance of an engine, to reduction of vibrations and noise and to a maintenance-free feature.
For example, an apparatus for an OHC (overhead camshaft) engine for use with an automobile or a like vehicle has been developed wherein a valve operating system for operating an intake valve or an exhaust valve is operated to vary the operating timing of the intake or exhaust valve in order to enhance the performance of the engine.
In the apparatus of the type described above (that is, variable valve timing mechanism), for example, a cam for a high speed and another cam for a low speed are provided on a camshaft and selectively used to obtain an operation timing of the intake or exhaust valve in accordance with an operating condition of the engine.
The high speed cam has a cam profile which can provide an operation timing suitable for high speed operation, and the low speed cam has another cam profile which can provide another operation timing suitable for low speed operation.
In a cam apparatus of the rocker arm type, the selection mechanism between the high speed cam and the low speed cam is constructed such that a pair of rocker arms are selectively connected to or disconnected from each other so that the valve is operated alternatively by the high speed cam or the low speed cam in order to obtain an operation timing of the intake or exhaust valve in accordance with an operation condition of the engine.
In order to allow adjustment of the clearance between a rocker arm and a valve, generally an adjust screw (tappet screw) is provided at a valve contacting portion of the rocker arm at which the rocker arm contacts with the valve.
However, where such adjust screw is employed, a gap may possibly be produced between the adjust screw and an opposing end of the stem of the valve, and if such gap exists, then when the valve is operated by rocking motion of the rocker arm, the adjust screw hits the end of the valve stem to produce an impact sound. Particularly with a valve operating system with a variable valve timing mechanism, if the adjustment of the clearances between rocker arms and valves is not performed appropriately, when it is tried to connect the rocker arms to each other, a required movement may be obstructed to obstruct smooth adjustment of the valve timing.
In order to eliminate this, the clearance between the rocker arm and the valve must always be controlled to adjust the position of the adjust screw in accordance with the degree of abrasion at the end of the adjust screw and so forth.
In order to achieve reduction of vibrations and noise and a maintenance-free feature of a valve operating system with a variable valve timing mechanism, a valve operating apparatus has been proposed in Japanese Patent Laid-Open Application No. Showa 61-81510. In the valve operating apparatus, a hydraulic lash adjuster (HLA) is incorporated in a valve contacting portion of a rocker arm in place of an adjust screw so that the clearance between the rocker arm and the valve may be adjusted automatically.
In the valve operating mechanism, however, since the hydraulic lash adjuster is disposed at the valve contacting portion of the rocker arm, the valve side conversion weight of the valve operating mechanism is increased, which deteriorates the operating characteristic of the valve so that the valve may possibly not operate in a designed manner.
Consequently, the valve operating mechanism has a subject particularly with regard to output power of the engine during high speed rotation and durability of the engine.
Further, since the valve contacting portion of the rocker arm is spaced away from the center of rocking motion of the rocker arm, also the hydraulic lash adjuster located at the valve contacting portion is spaced away from the center of rocking motion of the rocker arm. This leads to the following drawbacks.
In short, particularly during high speed rotation of the engine the hydraulic lash adjuster is acted upon by an acceleration and a centrifugal force caused by rocking motion of the rocker arm so that a check valve ball built into it is liable to rage, and consequently, oil in a high pressure chamber in the hydraulic lash adjuster may flow out to increase the valve lift loss. Accordingly, the subject with regard to the output power and the durability of the engine becomes progressively important.
Naturally, when it is tried to solve the subject, a possible increase of the size and/or the weight of the valve operating system is desired to be suppressed as small as possible.
Thus, it seems a promising idea to provide, separately from a rocker arm, an arm (swing arm) which can be adjusted in phase with respect to the rocker arm such that the swing arm is contacted with a valve and a hydraulic lash adjuster is interposed between the rocker arm and the swing arm such that it can automatically adjust the relative phase between the rocker arm and the swing arm.
In this instance, however, since the valve operating system includes an increased number of components, the following subjects are produced depending upon the arrangement of the components:
1. Where a cam for a low speed and another cam for a high speed are disposed at a comparatively short distance, when the cams are to be cast together with the cam shaft, the mold drawability is deteriorated.
2. Where the low speed cam and the high speed cam are disposed at a comparatively short distance, when the low speed cam is to be polished, the grind stone may possibly interfere with the high speed cam which projects rather than the low speed cam.
3. Where the valve operating system is, for example, of the two valve type, the overall width of the valve operating system may be increased, resulting in decrease of the degree of freedom in designing.
4. Upon high speed operation, a lift of the high speed cam is transmitted from the rocker arm for a high speed (sub rocker arm) to the rocker shaft by way of a piston (plunger) and then transmitted from the rocker arm for a low speed (main rocker arm) to the valve by way of the swing arm. Upon such transmission of power, the rocker shaft undergoes torsion, which is a cause of deterioration of the rigidity of the entire valve operating system.
Further, where a hydraulic lash adjuster is employed for a valve operating system as described above, this has a significant influence upon the motion characteristic of the valve operating system.
On one hand, the rigidity of the entire valve system is deteriorated since the high pressure of oil in a high pressure chamber acts upon the valve system. As a countermeasure for this, enhancement of the rigidity of the high pressure chamber is required by increasing the diameter of the plunger and decreasing the volume of the high pressure chamber. In this instance, however, the diameter of the plunger has a higher influence.
On the other hand, although depending upon the type of the valve operating system, the valve side conversion weight is increased by attachment of the hydraulic lash adjuster.
Further, since the inertial weight around the valve and the valve spring force act directly upon the hydraulic lash adjuster, the hydraulic lash adjuster must necessarily have a correspondingly high rigidity.
Thus, if the diameter of the plunger of the hydraulic lash adjuster is increased in order to enhance the rigidity of the high pressure chamber, then this results in an increase of the weight of the hydraulic lash adjuster and deterioration of the motion characteristic of the valve. Consequently, the subject described above becomes further significant.
Therefore, it is desired to reduce the inertial weight around the valve and the valve spring force which act upon the hydraulic lash adjuster by some means.
Meanwhile, in a valve operating system with a variable valve timing mechanism, it is desired to keep the valve clearance at an appropriate value. Particularly, the end of the stem of the valve and the contacting portion of the rocker arm with the end of the stem of the valve are liable to be abraded, and such abrasion may obstruct appropriate operation of the variable valve timing mechanism.
Particularly where the hydraulic lash adjuster is incorporated in the valve operating system as in the valve operating apparatus disclosed in Japanese Patent Laid-Open Application No. Showa 61-81510 mentioned above, the load to the valve operating system is increased, and there is a subject that the abrasion at the contacting portion described above is liable to increase.